Transposition is a genetic rearrangement process in which DNA sequences termed transposable elements "move" from one DNA location to another. A molecular understanding of transposition is important because: similar DNA rearrangements are likely to have played an important role in genome evolution and in the somatic cell chromosome changes associated with some cancers, retrovirus genomes integrate into host chromosomes by a transposition-like process, the V(D)J breakage reaction is catalyzed by a transposition-like process, and bacterial transposable elements frequently carry (and help disseminate) antibiotic resistance genes. Furthermore, these studies will elucidate how a single polypeptide, the Tn5 transposase, is able to perform the multiple sequential functions involved in transposition. We will use the bacterial transposon Tn5 to study the molecular mechanisms involved in transposition. Tn5 is the best model for studying DNA transposition because our studies have resulted in the development of a unique structural framework; a molecular structure of the complete transposase in complex with Tn5 end DNA sequences. No comparable information is available for any other transposase or retroviral integrase. In addition we have accumulated a substantial body of biochemical and genetic information to help elucidate the many steps involved in the transposition process. During the next grant period we will extend our genetic, biochemical and structural studies of Tn5 transposition.